I have an '04 Blazer. I just finished a heater core install & all went well. All the heater selections work fine except the defroster. It stays on defroster/floor. What do you think - stuck door, vac leak, vac actuator??

 

Chances are its a vac leak somewhere! I have a small leak and mine will sometimes not switch until I get out on the road and accelerate creating more vacuum. It will sometimes bounce back and forth as well while driving. This will all be fixed in a week or two once I get the lines swapped out finally!